b. in London 26 May 1809, he was the son of George Clark (1777 - 1848), a chaplain of Chelsea Hospital, and Clara Dicey; Samuel Clarke, the theologian, was his great-grandfather. He was educated at Charterhouse and after engaging for some time in medical studies qualified as an engineer. He worked (under Brunel) on the Great Western Railway (he published in 1839 an anonymous guide to the G.W.R., expanding it in 1846), and for a time in India. Later, he became inspector under the short-lived General Board of Health (his reports on various towns included some in South Wales), and afterwards one of the three commissioners.

In 1852 he accepted the position of trustee of the will of Sir Josiah John Guest, and from then until 1897 he was the effective controller of the famous Dowlais iron-works. The business was then working at a loss, but Clark raised it to a leading position in the country, and so maintained it. His aptitude for far-sighted innovation was as pronounced as his administrative skill. He assisted Bessemer to perfect his process; he acquired iron-ore deposits at Bilbao and undeveloped coal areas in Glamorgan; and finally moved his main works to Cardiff to economize on inland transport. His interest in social welfare was as great as in his business. The Dowlais hospital was erected at his expense; the schools, then the largest in the kingdom, were due to his initiative; and he was the universal chairman of the local authorities.

His more lasting title to remembrance rests upon his archaeological work, the product of leisure hours. An account of Caerphilly castle written in 1834 was a remarkable production for a man of 25. In 1843 he helped to found the society afterwards called the Royal Archaeological Institute, and was associated actively with that body and the Cambrian Archaeological Association during the whole of his life. As an engineer, he was attracted to the subject of medieval fortification, and wrote careful studies of individual castles at intervals. These studies were collected in 1884 in his Medieval Military Architecture with an introduction setting out his considered views on the subject. Later research has invalidated some of his theories, but nothing can ever invalidate his scrupulously accurate descriptions. His great collection of the Glamorgan charters (2nd ed., six vols., 1910, entitled Cartae et Alia Munimenta quae ad Dominium de Glamorgancia pertinent…) has been the foundation of all subsequent work on the history of the county, and was the foundation of his own Land of Morgan, 1883. The thick volume of Glamorgan pedigrees (Limbus Patrum Morganiae et Glamorganiae), 1886, is another monument of painstaking research. In short, there can have been few migrants who identified themselves so completely with the land of their adoption.

Clark m. Ann, daughter of Henry Lewis of Greenmeadow, Tongwynlais, 3 April 1850; she died 6 April 1885, leaving a son Godfrey Lewis Clark (died 1924) and a daughter. Clark d. at his home, Tal-y-garn, near Pont-y-clun, 31 January 1898.